A terminal can communicate with a central processor only if it operates in accordance with the communications protocol that applies to that processor. In simple terms, the communications protocol is a set of rules defining the format, control and sequencing of message transfers within the computer system. Since computer manufacturers have not agreed to adopt a single, standard protocol defining the rules of communication over their computer systems, a number of different protocols are used. As a consequence, it is not uncommon for the equipment made by one manufacturer to be unable to communicate directly with the equipment made by another manufacturer. It is, however, often desirable, particularly from the user's point of view, that such communication be possible.
Indeed, if two central processors are available, each one on its own network and each one using a communication protocol which is different from the other processor, it may also be desirable that a terminal be capable of switching from one network to the other network while maintaining active sessions on both of them. A switching circuit may be required to provide such capability. However, the switching circuit must be able to switch the terminal from one network to the other network without disrupting communications on either one. Thus, for example, it must not generate noise or other signal transients which could be misinterpreted by either network and acted on as though they were valid messages. And it must switch quickly so as not to interfere with subsequent messages or control signals sent over either network.